You may very well be right on TTL option, though the demand is low for that on a housing made for a camera primarily used to shoot video. I'm sure stills will be great on the GH5, so maybe an add on option for either (e.g $200 for regular flash trigger, $400 for TTL).

Edit: Why am I offering up money making options for Nauticam? Move along, nothing to see here...

hmm regarding the fiber optic cable..which one are you using now ? got iDive ones and they work fine with built in flash but dunno about the trigger

I had issues with my old fiber optic cables initially. We tried swapping the LEDs on the flash trigger for some ultra-bright ones, and that didn't work either.

The expensive Sea&Sea cables do work though.

And, after some hassles (they sent the the wrong ones, and still charged me for them), I got a set of cheaper ones from Divervision. I like these ones because they're straight not coiled, and I can have a more streamlined rig.

Just make sure that whatever cables you get have 613 cores, and I think you should be good. With 613 cores, there's noticeably more light transmitted than the crappy single-core variety.

I use the Panasonic DMW-FL70 Flash in my Nauticam housing on a GH-5. It works very well in TTL and manual. Don't know if it will fit in your housing though. If you want, I can take measurements for you.

Drawback is that it has no batteries, so you will drain the batteries of your camera faster. I had no problems during 2 -3 hour dives however.